/Engine/Plugins/Experimental/Fracture/Source/FractureEngine/Public/FractureEngineClustering.h
Merge selected clusters. Non-clusters in the selection are converted to the closest (parent) clusters. On success, returns true and InOutSelection holds the index of the cluster to which the selection was merged.
static bool MergeSelectedClusters ( [FGeometryCollection](API\Runtime\Chaos\GeometryCollection\FGeometryCollection) & GeometryCollection, [TArray](API\Runtime\Core\Containers\TArray)< int32 > & InOutSelection )